A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus-described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate, processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning structure” used herein should be broadly interpreted as referring to structure that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
Patterning structure may be transmissive or reflective. Examples of patterning structure include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure supports, i.e. bears the weight of, the patterning structure. It holds the patterning structure in a way depending on the orientation of the patterning structure, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning structure is held in a vacuum environment. The support may include mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning structure is at a desired position, for example, with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning structure”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory actions may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
In order to accurately apply a desired pattern onto a target portion of a substrate, the reticle should be aligned with respect to the substrate. Therefore, according to the prior art, the relative position of the reticle with respect to the substrate is set accurately, by measuring and adjusting the relative position. Alignment of the patterning structure with respect to the substrate is, according to the state of the art, for instance done using two alignment actions.
In the first action the substrate is aligned with respect to the substrate stage carrying the substrate, while in the second action the reticle is aligned with respect to the substrate stage. As a result of these two actions, the reticle is aligned with respect to the substrate, as desired.
In case a single stage machine is used, the first and second action are carried out at the exposure position. In case a dual stage machine is used, the first action may be carried out at a first position, remote from the exposure position. Then, the substrate stage with the substrate positioned on it is transported to the exposure position, where the second action is performed.
The first action may be carried out with two sensors. A first sensor measures the relative position of the substrate with respect to the substrate stage in x, y and Rz directions, where the xy-plane is defined as the plane that is substantially parallel with the surface of the substrate, the x- and y-direction being substantially perpendicular with respect to each other. The z-direction is substantially perpendicular with respect to the x- and y-direction so Rz represents a rotation in the xy-plane, about the z-direction. A more detailed description about this sensor is, for instance, provided in EP 0 906 590 B1. A second sensor, usually referred to as the level sensor, measures the height of the surface in dependence on locations on the substrate to be exposed, creating a height map and based on the determined heights, also determines the rotations about the x and y-axis: Rx, Ry.
Next, in the second action, the reticle is aligned with respect to the substrate stage. This may be done with an image sensor, such as a transmission image sensor (TIS), as will be known to a person skilled in the art. A TIS measurement is performed by imaging a first alignment pattern provided on the reticle (mask alignment mark) through the projection system (lens) to a second alignment pattern provided on the substrate stage. The alignment patterns comprise a number of isolated lines. Inside the substrate stage, behind the second alignment pattern a light sensitive sensor is provided, e.g. a diode, that measures the light intensity of the imaged first alignment pattern. When the projected image of the first alignment pattern exactly matches the second alignment pattern, the sensor measures a maximum intensity. The substrate stage is now moved in the x- and y-direction on different z-levels, while the sensor measures the intensity. Therefore, the TIS is actually an aerial image sensor, in which multiple scanning slits probe the aerial image of isolated lines. Based on these measurements, an optimal relative position of the substrate stage can be determined. The TIS sensor will be explained in further detail below with reference to FIG. 2. It will be understood that instead of a transmission image sensor, also a reflective image sensor may be used. In this case the second alignment pattern provided on the substrate stage is reflective, and the light sensitive sensor is not positioned inside the wafer stage. Therefore it will be understood that although the text refers to transmission image sensors, this may in general be any type of image sensor.
However, the inventors have discovered that the use of TIS measurements involves a few disadvantages. The position of the isolated lines is defined by the reticle, the illumination mode applied and the lens aberrations. The first and second alignment patterns have different structures and dimensions than the features of the patterning structure to be imaged on the substrate. The features of the patterning structure may have a different aligned position than the alignment pattern, e.g. due to a different response to lens aberrations. Thus, the lens may treat them differently, i.e. may project the alignment pattern on a different location than the structure of the patterning structure. This may cause an offset in the aligned position, in the lateral direction (in x, y plane) as well as in the axial direction (z-direction).
An alignment sequence usually consists of multiple single point alignments. The result of a single point alignment equals an aligned position x, y and z. When multiple points are aligned, the rotation can be computed based on the relative positions between the points. Since aberrations differ for different positions, the single point alignment by TIS is influenced differently for different positions. So, offsets in all parameters may be expected.
It is also possible to perform the alignment in one action, instead of in two actions as described above. In such a single step alignment scheme, the reticle is directly aligned with respect to the substrate. This is done using a through-the-lens alignment system measuring in the x, y and Rz directions, of which a more detailed description is provided in EP 1 026 550 A2.
As already stated above, the aligned position measured by TIS is influenced by lens aberrations. When increasing the resolution of a lithographic system without changing numerical aperture (NA) and wavelength, new illumination modes are introduced (e.g. low k1 factor). These modes have in common that the light is coming from a limited number of small angles (poles). As example, dipole illumination illuminates the reticle under two angles with very small opening angles. These types of illumination modes also have a very discrete use of the lens NA. Using such different illumination mode types may further add to inaccurate alignment.
During exposure, the lens of the projection system will heat up, as a result of energy absorbed from the projection beam. Since a heated lens may have different optical characteristics, the lens aberration may change. The calibrated alignment is no longer valid, since it was performed using a ‘cold’ lens. Also this may add to inaccurate alignment.